An operating device for supplying one or more light emitting diodes is known from WO 2013/152368 A1. Such operating devices are primarily used for providing a desired power supply for the lamp, e.g. a light emitting diode (LED) or numerous light emitting diodes connected in series and/or in parallel. Additional functions, such as dimming the lamp, changing/adapting the luminous color, or compensation for fluctuations in the input voltage, may be provided in the operating device.
The light emission of a light emitting diode depends on the current flow through the light emitting diode. To control or regulate the brightness, light emitting diodes are thus operated in a mode in which the current flow through the light emitting diode is controlled or regulated by the operating device.
Switching regulators can be used to activate light emitting diodes, e.g. step-up converters or step-down converters, also referred to in the field as boost converters or buck converters, respectively. In a switching regulator of this type, a control device activates a quick switch, which causes current to flow through a coil (as well as through the light emitting diode, with a buck converter) when it is switched on. After switching it off, the coil continues to drive the current (in the case of a buck converter) through the light emitting diode.
A number of measurement values are determined by the control circuit of the switching regulator for control or regulation, in order to counteract deviations from predefined operating parameters, such as luminous color and brightness, or shifts in the light spectrum at different dimming levels.
For the determination of the measurement values and the generation of corresponding signals, which can be supplied to and processed by the control circuit, additional components/circuits are needed, making the overall construction of the switching regulator complex and expensive.
Depending on the application, the switching regulator is operated in a continuous operating mode, in which the switch is switched back on before the current through the coil has decreased to zero, in a limit operating mode, in which the switch is switched back on as soon as the coil current has decreased to zero or the zero line has been reached by a positive side, or in a discontinuous operating mode, in which the switch is not immediately switched on again as soon as the coil current has decreased to zero or the zero line has been reached with a positive side.
In the continuous operating mode (gap-less operation) and in the limit operating mode, a change in the power output of the switching regulator can take place very easily via a change in the threshold value of the coil current at which the switch is switched off. Problems may arise, however, with low dimming levels or low output currents to the light emitting diodes.
Thus an arbitrary decreasing of the switching-off threshold in the limit operating mode is frequently not possible, because problems may arise with low thresholds regarding the detection and processing of the low current value. Furthermore, a large coil is needed for the continuous operating mode for low output currents, making the converter expensive, and increasing its dimensions.
Sheng Liu et al.: “An Adaptive On-Time Controlled Boost LED Driver with High Dimming Ratio,” IECON 2012—38th Annual Conference on IEEE Industrial Electronics Society, vol., no., pp. 210, 214, Oct. 25-28, 2012, describes an operating device for light emitting diodes that functions with pulse modulation in a continuous operating mode with high load requirements or marginal dimming, and in a discontinuous operating mode with low load requirements or greater dimming. A precise control/regulation of the brightness is also possible with low load requirements with this operating device, or boost converter. It is also not necessary to have a large coil for low output currents.
However, in order to control the switching between the operating modes, and in order to implement the pulse frequency modulation, additional components/circuits are needed with the operating device described therein, making the overall construction of the operating device complex and expensive.